Oblique magnetic track recording is known to be particularly suitable for the stationary reproduction of single pictures, because a picture field or a picture frame can be recorded in one oblique track. The only difficulty which occurs results from the differences in inclination between the video heads which scan during tape stops and the magnetic tracks which are recorded at normal tape speed.
As will be described in further detail hereinafter, oblique tracks can be recorded on a magnetic tape at normal tape speed and scanned at tape stop. These tracks may occupy favorable or unfavorable positions with reference to signal-to-noise ratio. This is because the areas which overlap the space between the tracks fall on the picture edges or on the vertical blanking intervals.
Establishing a favorable track position manually by back-and-forth rotation of the tape reels can only be accomplished with much labor, and sometimes is practically impossible such as, for example, with coaxial reel arrangements. An arrangement for automatic track searching has already been disclosed in a publicly available German application (DT-OS No. 2,115,457). In this arrangement, temporal coincidence is determined between the occurrence of the scanned maximum of the envelope curve and the occurrence of a vertical synchronizing pulse. With the aid of control logic for the tape transport motor, provision is made, by means of one or several back-and-forth motions, such that this places the maximum of the envelope curve into the temporal middle of a picture field. In other words, the optimum position of a scanning track is achieved by causing the maximum of the envelope curve to coincide with half the time interval between two vertical pulses.
This process can clearly function satisfactorily only if the recorded oblique tracks actually are ideal straight lines. However, in general, this is not the case because minor variations in tape stretch or other factors cause a slight wave-shape or crooked course in the recorded oblique track.
For this reason, the maximum of the envelope curve does not occur exactly at the track center, but, on the contrary, is somewhat displaced. In this case and when the automatic track search of the prior art forces a coincidence between the maximum of the envelope curve and the temporal middle of a picture field, the scanned voltage of the envelope curve may have its minimum within the visible video picture.